1. Field of the Invention
The present invention relates to a process for producing porous silicone moldings and also to the moldings obtainable thereby and to their use.
2. Description of the Related Art
Porous silicone moldings are used in many sectors, for example as sealing material in vehicle construction or as insulating material in the building industry. A molding is a three-dimensional construct, preferably a foil or a fiber, and the thickness of the molding normally does not exceed 10 mm.
Porous silicone moldings are usually produced using two-part systems comprising a hydrogenmethylpolysiloxane and a vinylpolysiloxane. Adding a hydroxide source produces hydrogen, which leads to the actual foaming. Such a process is described in DE 1224040 for example. This process usually produces thick porous silicone moldings of open-cell or closed-cell structure, but is unsuitable for producing thin porous silicone moldings, for example membranes.
Membranes are thin porous moldings and are used to separate mixtures. They are further used in the textile sector, for example as breathable and water-repellent membranes. One advantage of membrane separation processes is that separation can be carried out even at low temperatures, such as room temperature for example, and therefore have lower energy requirements compared with thermal separation processes, such as distillation.
Phase inversion by evaporation is a known way to process cellulose acetate or polyvinylidene fluoride into thin porous moldings. This process does not need a coagulation medium or an additional foaming reaction. In the simplest case, a ternary mixture is prepared from a polymer, a volatile solvent and a second, less volatile solvent. Following wet film formation, the volatile solvent evaporates, causing the polymer to precipitate in the second solvent and form a porous structure. The pores are full of the second solvent. The second solvent is subsequently removed from the molding, for example by washing or evaporation, to ultimately obtain a porous molding. EP 363364 for example describes the production of porous PVDF moldings on the basis of this process.
The use of a phase inversion process for silicones is unfamiliar to a person skilled in the art, since any pores actually formed in the course of evaporation normally collapse again owing to the silicone still being flowable, and hence the molding loses its porosity.
The production of porous silicone moldings by the Loeb-Sourirajan process is known. JP 55225703 for instance teaches the production of a porous silicone membrane comprising a silicone-carbonate copolymer. This process exclusively provides an anisotropic pore size along the film layer thickness. In addition, a separate coagulation bath is also required at all times. It would be advantageous to be able to use other silicones or silicone copolymers to produce thin porous moldings in addition to the silicone copolymers mentioned in this patent specification. The production of isotropic porous silicone moldings would likewise be advantageous.